If fuel cells are to become viable portable power sources in the future, solutions to a number of difficult, persistent technical problems are needed. Most of these problems are associated with the presence of the proton exchange membrane, which is highly sensitive to various factors, such as operating temperatures and membrane humidity. Efforts in portable applications have largely focused on reducing the size of proton exchange membrane (PEM) fuel cells. By portable power sources, this is generally referring to substitutes for batteries that power portable electronic devices. This approach carries all the cost and efficiency issues associated with larger scale PEM fuel cells. Moreover, the reduction in size exaggerates some of these problems, and introduces even further problems that require resolution for a commercially viable product.
One approach has been to deliver laminar flows of oxidizer and fuel saturated electrolytes into a single channel with a cathode on one side and an anode on another. See, e.g., Membraneless Vanadium Redox Fuel Cell Using Laminar Flow, Ferrigno et al., J. Amer. Chem. Soc. 2002, 124, 12930-12931; Fabrication and Preliminary Testing of a Planar Membraneless Microchannel Fuel Cell, Cohen et al., J. Power Sources, 139, 96-105; and Air-Breathing Laminar Flow-Based Microfluidic Fuel Cell, Jayashree et al., J. Am. Chem. Soc., 2005, 127, 16758-16759. See also, U.S. Pat. Nos. 7,252,898 and 6,713,206. Each of these is incorporated into the present application by reference in their entirety for background teachings.
This approach has various shortcomings. First, the fuel and oxidizer will mix downstream of the entry point, wasting the majority of the fuel. This is inefficient. Second, the diffusivity of many oxidizers leads to mixed potentials at the anode due to oxidizer crossover to the anode. This takes energy away from the circuit and also leads to inefficiency of the overall cell. Third, the architecture of the cell is restricted to the geometries, lengthscales, and electrolytes where laminar flow is ensured.
U.S. Patent Publication Nos. 2003/0165727 and 2004/0058203 disclose mixed reactant fuel cells where the fuel, oxidant and electrolyte are mixed together and then flow through the anode and cathode. These publications are incorporated herein by reference. According to these publications, the anode is allegedly selective for fuel oxidation and the cathode is allegedly selective for oxidizer reduction. The designs in these publications have significant shortcomings. First, the amount of some oxidizers that can be typically carried by an electrolyte is relatively low (e.g., the oxygen solubility in an electrolyte is typically quite low relative to fuel solubility). This means that a relatively high flow rate is required for the mixed reactants to ensure that an ample amount of oxidizer is flowing through the cell. That is, a relatively high flow rate is required to maximize oxidizer exposure and reaction at the cathode. But increasing the flow rate requires increased work, thus detracting from the overall power efficiency of the cell. Moreover, electrodes that are selective by virtue of their material properties tend to have lower reaction activity rates than non-selective electrodes. Because the designs in these two publications rely primarily on the use of selective electrodes for both the cathode and anode, this further detracts from the efficiency of the cell.
The present application endeavors to meet the long-felt and unresolved need for a fuel cell technology that eliminates the use of a proton exchange membrane, yet operates efficiently and effectively.